Why does dropping Mentos into a bottle of Diet Coke create a geyser effect?

Mostly because the Mentos provide nucleation sites for the dissolved carbon dioxide in the Diet Coke to escape as a gas. Other active ingredients in the cascade-effect reaction include aspartame (artificial sweetener), potassium benzoate (preservative), and caffeine in the Diet Coke; and gum arabic and gelatin in the Mentos. The ingredients seem to have a perfect compatibility with each other and, when mixed together and added to the nucleation, creates a chemical reaction that forces the soda to release all of its dissolved carbon dioxide at once, thus causing a more violent eruption than carbonated water alone.

(This is the first segment not to be assigned a “busted”, “plausible” or “confirmed” rating, as there was no “myth” to be proved or disproved. Adam and Jamie did the tests simply to dissect the process and determine what actually makes a so-called Diet Coke geyser work.)

A stamp placed on the rotor of a helicopter can unbalance the spinning rotors enough to cause it to crash.

busted

A stamp in a controlled scale test did not cause any changes to the helicopter rotor’s rotation. The MythBusters immediately ramped up the test to the scale of 8,000 stamps, which destroyed the scale model helicopter. The full-sized test also produced the same results as the stamp did not cause the helicopter to crash and, according to the pilot, did not cause any significant change in the way the helicopter flew. A stamp placed on the tail rotor also did not cause any noticeable change.